The invention relates to a method for determining the position of the peak of a pulse in a signal received at a receiver. The invention relates equally to a device and to a cellular communication system which realize this method.
The position of the peak of a pulse in a received signal can be required for example for determining the delay of the signal when propagating from a transmitting unit to the receiver.
The delay of signals at a receiver can be evaluated for instance by a location service for determining the current location of the receiver. In case of a line-of-sight transmission, the delay of a signal is directly dependent on the distance between the receiver and the respective transmitting unit.
Such a location service can be provided in particular by a CDMA (code division multiple access) based satellite positioning system or by a CDMA based terrestrial cellular positioning system. In a CDMA based system, a data sequence is used by a transmitting unit to modulate a sinusoidal carrier, and then the bandwidth of the resulting signal is spread to a larger value, e.g. by multiplying the modulated signal with pseudo-random bits derived from a CDMA spreading code. These bits are usually referred to as chips.
In a CDMA system, the searching procedure performed for detecting a delayed signal taking the shortest propagation path is normally carried out in the impulse response of signals received from different transmitting units. The delay can be estimated e.g. by an edge detection in the impulse response profile of the received signals. The length of the impulse response profile is much longer than the width of the signal shape. Therefore, the search for an edge is performed along the impulse response, started from a certain position of the signal by comparing the amplitude of sampling data with a pre-defined threshold. The edge detection is thus a hitting process. The threshold has to be set on the one hand high enough in order to avoid that a noise peak is detected as signal edge, which would result in a false alarm. On the other hand, the threshold has to be set low enough to guarantee that the signal edge is detected even if the signal strength is rather weak.
The delay of a signal can only be determined accurately when the exact position of the peak of a signal pulse is known since this is the only clear reference point in the pulse. The result of the hitting process, however, is usually a position on the left side of the signal peak, i.e. on the side which is closer to a delay of zero, since a pulse will usually be detected before its peak is reached. Thus, the error of the edge detection is negatively biased. The error is more related to the signal than to the SNR (signal-to-noise ratio), which means that the variance can be high. The hit will occur between close to the peak of the pulse for weak signals and close to the bottom of the pulse for strong signals.
In case of a sampling rate of 2 samples per chip, the delay estimation error can therefore range from 0.0 chips to −1.0 chips. The average error is then about −0.5 chip for a triangular shape or waveform of the pulse.
Since the error has a negative bias, the simplest way to reduce the error is to introduce a positive factor to compensate for the bias. If the signal level is higher, the error is also bigger. Therefore, the compensation factor should be adaptive to the signal level. Still, with such a general compensation, a significant average error remains.